Knowledge of dietary availability of selenium in man is limited due to lack of non-invasive experimental methods applicable to normal man. In this proposal, we will develop the method of stable (non-radioactive) isotope enrichment of the diet in connection with neutron activation analysis to permit accurate and non-hazardous measurement of selenium bioavailability in man. We will investigate, in pilot studies, various aspects of selenium absorption and excretory pathways in normal adults in order to demonstrate the feasibility of this non-invasive method in humans. We will compare bioavailability of three different forms of selenium (selenite, selenomethionine, and factor-III) by appropriately labelling with the stable selenium, 74Se. Our long-term goal is to develop methods for the study of mineral bioavailability in all human subjects, including children and pregnant women; and to understand in quantitative terms any relation between dietary availability of selenium and its role in etiology of diseases, especially cancers.